1. Field of the Invention
This invention relates to a radiation image storage panel and a radiation image read-out method, wherein the radiation image storage panel is utilized. This invention particularly relates to a radiation image storage panel suitable for an image read-out operation, wherein a radiation image stored on the radiation image storage panel is read out at least from the side of a substrate of the radiation image storage panel, and a radiation image read-out method utilizing the radiation image storage panel.
2. Description of the Prior Art
Radiation image read-out methods utilizing radiation image storage panels have heretofore been used widely. With the radiation image read-out methods, a radiation image storage panel (i.e., a stimulable phosphor sheet), which comprises a substrate and a layer of a stimulable phosphor overlaid on the substrate and on which a radiation image of an object, such as a human body, has been stored, is exposed to stimulating rays, such as a laser beam, which cause the radiation image storage panel to emit light in proportion to the amount of energy stored thereon during its exposure to radiation. The light, which is emitted by the radiation image storage panel when it is exposed to the stimulating rays, is photoelectrically detected and converted into an electric image signal. After the image signal has been detected from the radiation image storage panel, the radiation image storage panel is exposed to erasing light, which releases residual energy from the radiation image storage panel.
The image signal obtained from the radiation image read-out methods is subjected to image processing, such as gradation processing and frequency processing, such that a visible radiation image having good image quality can be reproduced and used as an effective tool in, particularly, the accurate and efficient diagnosis of an illness. The image signal, which is obtained from the image processing, is used for the reproduction of the visible image on a photographic film or on a cathode ray tube. Energy remaining on the radiation image storage panel is erased in the manner described above such that the radiation image storage panel can be reused to record a radiation image.
Novel radiation image read-out methods have been disclosed in, for example, Japanese Unexamined Patent Publication No. 61(1986)-86743 and U.S. Pat. No. 4,883,961. In the disclosed radiation image read-out methods, while an image signal is being detected from a radiation image storage panel on which a radiation image has been stored, the regions of the radiation image storage panel, from which the image signal has been detected, are erased one after another. The disclosed radiation image read-out methods have the advantage in that the operation for reading out a radiation image from a radiation image storage panel and the operation for erasing energy remaining on the radiation image storage panel can be carried out approximately simultaneously, and therefore the cycle time of the whole system can be kept short.
Also, techniques for detecting light emitted by two surfaces of a radiation image storage panel have been proposed in, for example, U.S. Pat. No. 4,346,295 and Japanese Unexamined Patent Publication No. 4(1992)-280060. With the proposed techniques, the substrate of the radiation image storage panel is constituted of a transparent material capable of transmitting light, which is emitted by the stimulable phosphor layer of the radiation image storage panel, and the light emitted by the stimulable phosphor layer is detected approximately simultaneously from the two surfaces of the radiation image storage panel. In this manner, the efficiency, with which the light emitted by the radiation image storage panel is collected, is kept high. Further, the image signal components of two image signals having been detected from the opposite surfaces of the radiation image storage panel are added together in a predetermined addition ratio, which image signal components represent corresponding picture elements on the front and back surfaces of the radiation image storage panel. In this manner, the signal-to-noise ratio is kept high.
In cases where the radiation image storage panel provided with the substrate, which is constituted of a transparent material, is utilized, it is also considered to read out the image information only from the back side (i.e. the side of the substrate) of the radiation image storage panel.
However, if the substrate of the radiation image storage panel is constituted of a transparent material such that the light emitted by the stimulable phosphor layer can pass through the substrate, the substrate will also transmit the stimulating rays. Also, the stimulating rays can propagate through the substrate. In cases where, the image information is read out with an image read-out system, such as a photoelectric read-out means, from the back side of the radiation image storage panel provided with the substrate, the stimulating rays, which have passed through the substrate, impinge as noise upon the image read-out system. Therefore, the signal-to-noise ratio cannot be kept high. Further, the stimulating rays propagate through the substrate and stimulate the stimulable phosphor located at regions other than the region of the stimulable phosphor layer, which is to be stimulated. As a result, light is unexpectedly emitted by the regions other than the region of the stimulable phosphor layer, which is to be stimulated. The light emitted unexpectedly will often impinge upon the image read-out system. In such cases, an image having good image quality cannot be reproduced from the image signal, which has thus been detected. Ordinarily, the image read-out system is provided with a stimulating ray cutting filter, which prevents the stimulating rays from entering the image read-out system. Therefore, it is possible to prevent the aforesaid stimulating rays from entering the image read-out system. However, with the stimulating ray cutting filter, it is not possible to cut the light, which is emitted by unexpected regions of the stimulable phosphor layer.
The substrate described above also transmits the erasing light. Also, the erasing light can propagate through the substrate. Therefore, with the technique wherein the image read-out operation and the erasing operation are carried out approximately simultaneously, the erasing light propagates through the substrate. Therefore, as in the case of the stimulating rays described above, the erasing light impinges as noise upon the image read-out system. Accordingly, the signal-to-noise ratio of the detected image signal cannot be kept high. Further, the erasing light stimulates the stimulable phosphor located at regions other than the region of the stimulable phosphor layer, which is to be stimulated. As a result, light is unexpectedly emitted by the regions other than the region of the stimulable phosphor layer, which is to be stimulated. Moreover, the erasing light erases the image information stored in the regions, from which the image information has not yet been read out.